(1) Field of the Invention
The present invention relates to mass flowmeters for precisely measuring the mass flow rate of a fluid flowing through a conduit.
(2) Description of the Prior Art
An example of such mass flowmeters is disclosed in Japanese Patent Publication No. 56-23094, which comprises heat sensitive coils having a large temperature coefficient and mounted at an upstream position and a downstream position of a conduit. The flow rate is measured by supplying constant current to the heat sensitive coils and detecting temperature distributions at heat sensitive parts which are variable with fluid flows. Another example is disclosed in Japanese Patent Publication Kokai No. 59-18423, in which a passing fluid is conditioned such as by adjusting fluid temperatures. The temperature of the fluid varies as a result of a heat exchange taking place when the fluid passes through the flowmeter. The flow rate is measured by displaying an amount of energy consumed at either the temperature adjusting stage or the temperature varying stage.
The former lacks in responsivity since the measurement is influenced by the speed of variations in the temperature distribution and by the heat capacity of the conduit and its coating. The latter, though higher in response speed than the former, has the zero point readily variable with variations in the ambient temperature and in the heat capacity of the fluid since its operating principle is the same as that of a hot-wire current meter. A temperature adjusting circuit may be provided to eliminate this disadvantage, but the effect thereby produced is not significant for the complicated circuit construction.
As a further example, there is a mass flowmeter, as shown in FIG. 3, which includes a compensation circuit 65 expressed in the following formula: ##EQU1##
(1) However, perfect temperature compensation is impossible since (Vu+Vd), strictly speaking, not only varies with temperature variations but also with flow rate variations.
(2) Temperature setting resistors 61 and 61' must be resistors with temperature coefficients close to zero. The heat sensitive coils Ru and Rd, therefore, are set to a fixed temperature (of 80.degree.-90.degree. C., for example). As a result, great signal noise occurs when the ambient temperature exceeds the temperature of the coils Ru and Rd or when there is little difference therebetween, which renders the flowmeter unusable.
(3) Similarly, since the heat temperature of the coils Ru and Rd must be set considerably higher than the ambient temperature, the flowmeter is not applicable to measurement of a highly reactive fluid G.
Meanwhile, a proposal has been made, as shown in FIG. 4, to provide constant temperature difference circuits Tu and Td including, connected in series to the heat sensitive coils Ru and Rd, temperature detecting resistors 11 and 11' having approximately the same temperature coefficients as the coils Ru and Rd, respectively.
According to this system, the upstream and downstream coils Ru and Rd have a temperature characteristic expressed in the following equation: EQU Ru=Rd=Ro (1+.alpha.t) (1)
where Ru and Rd are the resistances of coils Ru and Rd respectively, Ro is the resistance of the two coils Ru and Rd at 0.degree. C., t is a temperature of the coils Ru and Rd, and .alpha. is the temperature coefficient of the coils Ru and Rd.
The constant temperature difference circuits Tu and Td are designed to operate so as to establish the following equation (2): EQU Ru(Rd)=(11)=(11')=ARo (1+.alpha.T) (2)
where T is the ambient temperature, and A is greater than 1 (since Ru must be greater than (11) and Rd greater than (11')).
Next, when equation (1) is substituted into equation (2), then; EQU t={(A-1)/.alpha.}+AT (3)
Thus the coil temperature t is multiplied by A in response to variations in the ambient temperature T, which does not result in a constant temperature difference. This flowmeter still is incapable of accurate mass flow rate measurement.